A lithium-ion secondary battery is one type of nonaqueous electrolyte secondary battery, and is a secondary battery in which lithium ions in the electrolyte are responsible for electrical conduction. A lithium-ion secondary battery has excellent characteristics, such as high energy density, excellent charged energy-retention characteristics, and a small memory effect, i.e., apparent capacity reduction. Thus, lithium-ion secondary batteries are used in a wide range of fields, such as cellular phones, smartphones, personal computers, hybrid automobiles, and electric automobiles.
A lithium-ion secondary battery mainly comprises a positive-electrode plate, a negative-electrode plate, a separator that insulates the positive- and negative-electrode plates, and a non-aqueous electrolyte solution. The positive-electrode plate is obtained by forming a positive electrode mixture layer on a surface of a positive electrode core. This positive electrode mixture layer may be produced by applying a positive electrode mixture paste to a surface of a positive electrode core, followed by drying. The positive electrode mixture paste is obtained by mixing an electrode active material with a conductive paste containing a conductive auxiliary agent (e.g., carbon), a binder, and a solvent.
As described above, the production of a positive electrode mixture layer is performed by applying a positive electrode mixture paste to a surface of a positive electrode core. Thus, the positive electrode mixture paste, as well as the conductive paste, which is a component of the positive electrode mixture paste, is required to have a low viscosity. Under such circumstances, methods for adding a dispersant to allow a conductive auxiliary agent to be dispersed in a conductive paste or a dispersion liquid are known (Patent Literature 1 and Patent Literature 2). A method for using a specific vinyl alcohol-based polymer as a binder is also known (Patent Literature 3). Polyvinylidene fluoride is useful for incorporating into such a conductive paste as a binder for binding a metal oxide and a collector. However, particularly when a polyvinylidene fluoride-containing conductive paste contains a basic substance, problems such as viscosity increase and gelation of the paste occur during storage.